This invention pertains to fluid resistors in general, and more particularly to a fluid resistor with a capilliary passage of substantially constant depth and of a varying configuration for providing a controllable resistance path for fluid flow.
The need for fluid resistors is important in various types of control instruments for obtaining predetermined gains and time constants such as in fluid integrators, operational amplifiers, and the like. The resistance to fluid flow in a system can be modified by the substitution of fixed value fluid resistors or the use of variable fluid resistors. A variety of fixed and variable fluid resistors or pressure dividers have been provided in the prior art, however, such devices have been found to be expensive to manufacture, difficult to manufacture with a good yield of repeatability, and in the case of variable fluid resistors having a limited range of operation. One type of variable fluid resistor of the prior art is a needle valve wherein the flow of a fluid through an aperature is controlled by the depth to which a pointed valve stem is inserted into the aperature to reduce the size of the aperature. The needle valve has the disadvantage of being particularly susceptable to dirt and other impurities since a very small area of fluid flow is controlled. In addition to the foregoing, the needle valve requires a multi-turn screw arrangement requiring many turns of a control knob to span the entire control range of the unit. This is particularly troublesome when attempting to calibrate the control knob with a scale.
Other fluid resistors have been developed in the prior art, wherein a capilliary type passageway is provided by two elements, each having a flat surface which are abutted together in close contact to provide a fluid seal therebetween. One of the elements includes groove formed into the element so that the groove and the flat surface of the element define the capilliary passageway. The size of the passageway (cross-sectional area) and the length of the passageway determine its resistance to fluid flow. The shape of the passageway varies in accordance with the particular type of structure involved and can take almost any type of form i.e., straight, curved, or serpentine. A variable fluid resistor of this type is disclosed in the U.S. Pat. No. 2,236,084, entitled "Adjustable Flow Restrictor", issued on Mar. 25, 1941 to W. J. Brown. In the device disclosed in the patent, and various other prior art devices, the capilliary groove is formed in the flat surface of one of the abutting members. Since it is the cross-sectional area of the groove that is one factor in determining the resistance of the capilliary passageway, the forming aperature, i.e. cutting milling, etc. is required to control both the width and the depth of the groove to high tolerance. Since the groove has dimensions in the thousandths of an inch, it is extremely difficult to form such grooves with a high degree of accuracy and repeatability, particularly so when the groove is of the complex or serpentine configuration. Furthermore, the difficulty experienced in accurately forming these grooves limits the length to which such capilliary grooves can be manufactured on a repeatable basis and therefore limited the range of variable fluid resistors. Because of the difficulties involved in manufacture the cost, etc., and the wide range of resistance values needed to fulfill control requirements, a plurality of variable fluid resistors are now provided, each having a different range that are to be substituted in the event that the system time constants or requirements exceed the resistance range of the particular device used.
A U.S. Pat. No. 3,532,127, entitled "Variable Fluidic Resistor Device", issued on Oct. 6, 1970 to T. H. Vogelsang et al, discloses a fluid resistor in which the the capilliary passage was provided by a laminate having a slot cut therethrough positioned between the flat surfaces of a pair of planar members, wherein the thickness of the laminate determines the depth of the capilliary passageway. With such an arrangement, the depth of the capilliary groove is fixed and the width of the groove is cut in accordance with the fluidic resistance requirements. With the laminate, the width of the slots cut therethrough (in the tens or hundredths of a thousandths of an inch) were generally greater than the slot depth (laminate thickness). As a result, the slots were required to be cut through very thin material. This presents problems not only in the required precision machining but also in the handling assembly and cleaning of such devices. Any dend, bend, etc., in the thin laminate material because of faulty handling, or machining, or the like, could destroy the laminate.
When the fluid resistors are to be variable, either the element having the capilliary groove, or its abutting element, is moved, or rotated, to control or select that portion of the capilliary passageway to be used for fluid flow. In such case, it is highly desirable that a lubricating tight sealing arrangement be provided so that wear and tear is minimized. This wear and tear between the abutting surfaces is particularly troublesome since large forces are required to be exerted on the planar members to assure that a good seal is achieved and leakage is minimized. It is therefore very important that the capilliary grooves are formed with smooth edges so that the abrasive effect of the movements between abutting surfaces of the planar members is minimized. This is particularly true when one of the flat planar members is made of valve sealing material to provide the desired degree of sealing and lubrication. Any rough edge of a groove tends to cut into the sealing planar surface and particles of valve material may fall into the groove and block or clog the capilliary passageway, thereby rendering the device useless. It would be highly desirable if a fluid resistor could be provided that includes a capilliary passageway that can have any variety of configurations without the need for precision forming of the depth of the capilliary passageway, and without creating rough edges. It would also be highly desirable if such fluid resistors could be manufactured on a mass production basis with a high degree repeatability in the operating characteristics.
It is therefore an object of this invention to provide a new and improved fluid resistor and the like.
It is also an object of this invention to provide a new and improved variable fluid resistor.
It is another object of this invention to provide a new and improved fluid resistor that can be manufactured by low cost mass production techniques with a high degree of repeatability.
It is also an object of this invention to provide a new and improved fluid resistor in which precision machining or cutting is not required to generate the capilliary groove, and the capilliary groove can be formed with smooth or rounded edges.
It is also an object of this invention to provide a new and improved fluid resistor wherein the capilliary groove has a substantially constant depth and wherein only the width and length of the groove need be controlled in the manufacture thereof.